This is a continuation-in-part of U.S. application Ser. No. 884,295, filed Mar. 6, 1978, now abandoned.
1. Field of Invention
This invention relates to turbine driven air cycle refrigeration systems for providing cooling air to aircraft cabins or other loads, and more particularly to a system in which a large volume of air recirculated from the cabin by a fan or ejector is used to prevent icing in the turbine discharge and downstream ducting, permitting the use of an ice-free regenerative heat exchanger to condense moisture from air entering the turbine. Improved cycle efficiency is attained by effective utilization of the recirculation means, a design which provides the maximum possible ventilation rate to the load, and the ability to efficiently utilize high supply air pressures.
2. Description of the Prior Art
As the cost and availability of fuel become significant factors in the operation of aircraft, improved efficiency of operation of aircraft components is more desirable in order to conserve available energy. One of the systems in modern day aircraft which consumes significant power is the air cycle environmental control system. Increased efficiency in this system will result in large fuel savings. A major obstacle to improving air cycle system efficiency has been the clogging produced by ice which forms in the turbine discharge and downstream ducting of the air cycle refrigeration systems. The maximum supply pressure level which could be used was that pressure which produced icing at the turbine discharge, and the cooling provided by any higher pressure was of necessity wasted because of de-icing controls required to prevent the ice. Consequently, all the pressure energy available in the compressed supply air could not be efficiently utilized due to the icing problems.
In an attempt to overcome limitations of the air temperature at the turbine outlet to prevent the formation of ice, some current air cycle refrigeration systems recirculate a small amount of air from the load and mix the recirculated air at a junction with the turbine discharge. A system such as this is presently incorporated in the "S-76" helicopter produced by the Sikorsky Aircraft Division of United Technologies Corporation. This approach has the advantage of being able to utilize very cold turbine discharge temperatures to reduce the required flow of compressed air, and also provides maximum possible ventilation rate through the load since the full sum of the recirculation flow plus the compressed air supply flow is used for cabin ventilation. One disadvantage of a system of this type is its reliance on simple scuppers at the turbine inlet and in the load distribution line to remove moisture from the air supply, a technique which is inadequate to remove large quantities of moisture.
Other air cycle refrigeration systems utilize a regenerative heat exchanger in the supply air path upstream of the turbine inlet to condense moisture at the turbine inlet and thereby avoid problems caused by the conventional coalescer-type water separator. A regenerative heat exchanger of this type is used in the Bell "Huey Cobra" helicopter having an air conditioning system manufactured by AiResearch. A disadvantage of this type of system is its inability to utilize very cold turbine discharge temperatures due to icing problems in the regenerative heat exchanger.
U.S. Pat. No. 4,127,011 uses recirculated air to raise the temperature of the turbine outlet air, a water extractor, and a regenerative heat exchanger for additional cooling of the supply air, and avoids the formation of ice at the outlet of the expansion turbine by heating the outside of the turbine wall with supply air. This system, however, does not provide maximum ventilation rate to the load since a portion of the turbine outlet air is recirculated through the regenerative heat exchanger, thereby making the recirculation air power source unnecessarily large for a given ventilation rate.
Other current air cycle refrigeration systems utilize recirculation of air from the load which is mixed with the air provided by the compressed air supply prior to supply of the air to the load. This technique had the advantage of supplying a ventilation rate to the load equal to the sum of the compressed air supply and the recirculated air. A system of this type is incorporated in the Boeing 747 aircraft. An advantage of this system is that maximum cabin ventilation rate is provided for a fixed recirculation fan capacity. Fully utilizing the recirculation fan flow reduces the parasitic heat load of the recirculation fan input power and therefore produces the maximum efficiency possible from the recirculation loop of the cycle. Unfortunately this type of system cannot fully utilize the total pressure energy in the pressurized supply air source because of the icing produced by the sub-freezing temperatures at the turbine outlet and downstream ducting.
Many current air cycle refrigeration systems use a de-ice control valve, modulated open as necessary, to supply warm air from upstream of the turbine inlet to the turbine outlet in order to prevent ice from collecting and causing blockage downstream of the turbine outlet. The Boeing 747 aircraft uses this type of system.
The present invention overcomes the limitations and disadvantages of the prior art air cycle refrigeration systems and provides a system having the highest possible efficiency in terms of maximum refrigeration and maximum ventilation for the load. The system may be designed for either more flow to the load for a given input power, or for the same flow to the load using less input power.